Screw air compressors are used in a variety of situations to compress air. One such use of screw air compressors is in the context of a nitrogen extraction system. In such a system, atmospheric air is initially compressed in the screw air compressor. The compressed air is then sent to an air separation system such as a membrane permeator, a pressure swing absorber, a cryogenic distillation column, or the like. Along the way, the compressed air can be conveyed through an appropriate cleaner to clean the air of condensate water, oil fumes and other contaminates. In the air separation system, nitrogen is separated from air to produce a nitrogen product having a desired purity, measured in terms of residual fraction of oxygen.
Generally speaking, in these types of systems which employ an air compressor, the desired purity of the resulting product dictates the amount of power consumed by the system, assuming a constant flow rate. That is, assuming power consumption is constant, more product (i.e., higher flow rate) can be produced with a lower product purity, or less product can be obtained with a higher product purity.
FIG. 1 illustrates some of the features and components of a known system for extracting nitrogen from compressed air. The system includes a screw air compressor 10 provided with an inlet 12 for allowing atmospheric air to be introduced into the casing of the compressor. In a known manner, a pair of rotors (not specifically shown in FIG. 1) are disposed within the casing of the screw air compressor 10 to effect compression of the air. Located downstream of the screw compressor 10 is a cleaner 14 for cleaning the compressed air of condensate water, oil fumes and other contaminates. Located downstream of the cleaner 14 is an air separation system which, as noted above, can be of various types such as a membrane permeator, a pressure swing absorber, or a cryogenic distillation column.
During operation, air is compressed in the screw air compressor and is fed downstream to the air separation system 16. The nitrogen which is then extracted from the compressed air in the air separation system 16 is directed downstream for use in the particular application for which it is intended.
With the compressor operating at full capacity, it oftentimes happens that the flow demand for extracted nitrogen decreases. In such a situation, continued operation of the screw air compressor 10 at full capacity will produce nitrogen having a purity greater than necessary. That is because the oxygen content in the nitrogen product will decrease. Depending upon the particular application, nitrogen having a product purity that is greater than required by specifications may be problematic. Even if the higher than necessary product purity does not present a potential problem for the particular use intended, it does nonetheless require expenditure of energy that exceeds the amount necessary for producing a product having a purity that is within specifications. Consequently, energy is wasted, and the life and longevity of the compressor is reduced.
One attempt to address this situation is illustrated in FIG. 1 and involves the use of an inlet control valve 18 positioned upstream of the inlet 12 to the screw air compressor 10 and an inlet control valve regulator 20. In this system, when the product (i.e., nitrogen) flow demand decreases, the system experiences an increase in pressure. This increase in pressure works its way back through the system where it is eventually sensed by the inlet control valve regulator 20. When the pressure increase exceeds a predetermined level, the inlet control valve regulator 20 causes actuation of the compressor inlet control valve 18. The compressor inlet control valve 18 is thereby throttled to restrict the volume of air entering the compressor. This then tends to reduce the amount of air compressed by the screw air compressor 10 in an attempt to match the demand, and prevents component damage caused by overpressure.
It has been found, however, that this type of air inlet control has little effect on the power consumed by the screw air compressor 10. That is, even though the flow rate through the system has been reduced as a result of restriction of the inlet air volume, the compressor continues to draw power at a level that is close to that consumed during full-load conditions. Thus, there is little energy savings with this type of system.
Another attempt at addressing the aforementioned situation involves designing the compressor to include a feature which will hereinafter be referred to as a sliding port 22. For purposes of simplicity and ease of understanding, the sliding port 22 is schematically illustrated in FIG. 1 as being separate from the screw air compressor 10. In actuality, as shown in FIG. 2, the sliding port 22 actually forms a part of the screw air compressor 10.
With reference to FIG. 2, the screw air compressor 10 includes a casing 24 in which is located a pair of rotors 26. The rotors 26 are schematically shown in FIG. 2. The wall of the casing 24 is provided with a series of longitudinally arranged orifices 28 that can be covered or uncovered by way of the sliding port 22. In operation, an actuator associated with the sliding port 22 actuates the sliding port 22 in order to open or close the orifices 28 in the casing wall. With all of the orifices 28 closed, the entire length of the rotors 26 serves to compress air flowing through the inlet 12. By appropriately operating the sliding port 22 to expose one or more of the orifices 28, air in the casing 24 is returned to the inlet 12 before being compressed. Thus, the effective length of the rotors which acts to compress air is reduced. This results in a reduction in overall power consumption of the screw air compressor.
With reference once again to FIG. 1, a discharge pressure control regulator 30 senses the compressor discharge pressure. When the nitrogen flow demand decreases, the resulting increase in pressure makes its way back through the system where it is eventually sensed by the discharge pressure control regulator 30. When the pressure exceeds a predetermined value, the sliding port 22 is actuated to expose one or more of the orifices 28 in the casing wall. In that way, the amount of air to be compressed is reduced, thereby resulting in a decrease in power consumption.
Some systems will also utilize the sliding port 22 and the control valve 18 in series with one another. For example, the system can be designed such that the sliding port 22 is initially actuated when the pressure in the system exceeds a first predetermined value. Thereafter, if the pressure in the system continues to increase due to a further reduction in flow demand, the inlet control valve 18 is actuated to modulate the volume of air entering the screw air compressor. If the pressure in the system further increases, the compressor can be designed to unload so that all of the air in the compressor is vented to the atmosphere so that the compressor stops compressing. Thereafter, the compressor can be shut down.
Other systems sometime employ a buffer tank downstream of the screw air compressor. If the flow demand decreases while the screw air compressor is operating at full load capacity, the associated increase in product purity is attenuated slightly by the buffer tank which is located downstream of the air separation system. That is, product in the buffer tank which possesses a purity that is within specifications is mixed with the product having a higher purity. In that way, the increase in product purity is not as noticeable to the customer.
Although being useful in some respects, the systems described above are subject to certain drawbacks and are susceptible of improvements. For example, the operation of the sliding port 22 and the inlet control valve 18 requires sensing of an increased pressure in the system. However, pressure increases which result from decreases in flow demand typically are offset by the then reduced performance of the air separation system before they can be sensed by the regulators 20, 30. Thus, changes in product flow demand cannot be accompanied by appropriate reductions in the operating level of the compressor to thereby achieve lower power consumption. Consequently, energy is still expended unnecessarily.
Needless energy consumption also occurs because changes in product flow demand are oftentimes not accompanied by immediate pressure increases. Rather, product purity typically increases until the product flow demand decreases to an extent that causes a pressure increase which can be sensed by the regulators 20, 30. Thus, product purity in excess of that necessary is produced and this, of course, results in excessive energy use.
Additionally, buffer tanks are oftentimes quite large--in some instances several times the size of the entire nitrogen generator system. If space is limited, it may not be possible to employ a buffer tank. Moreover, buffer tanks which are large in size can be quite sensitive to environmental factors such as wind and earthquakes. This is a particularly significant concern given the fact that the buffer tank stores a large amount of gas which, if released into the atmosphere, could cause significant concerns. Finally, buffer tanks can be quite expensive and may require extensive foundation work.
In view of the foregoing, there exists a need for a control system for controlling an air compressor, and a nitrogen extraction system which includes such a control system and air compressor for providing a real load-follow capability so that product purity can be maintained regardless of reductions in product flow demand. Further, there exists a need for an air compressor control system and a nitrogen extraction system including such a control system for permitting reduced power consumption at lower product flow demands while also maintaining a constant product purity. It would also be desirable to increase the life and longevity of air compressors by avoiding needless and excessive compressor operating levels, and/or needless stops and starts.